The invention relates to ion pumps used primarily in high and ultra-high vacuum systems.
Ion pumps are used in a variety of systems that require a high or ultra-high vacuum. Such systems include focused ion beam systems, electron microscopes, accelerators, molecular beam epitaxial deposition systems, and other analytical, fabrication and research systems and instruments. Ion pumps are typically used at pressures of between 10xe2x88x924 Torr and 10xe2x88x9211 Torr, with pressures of between 10xe2x88x927 Torr and 10xe2x88x929 Torr being common in, for example, focused ion beam systems.
One type of ion pump is the diode sputter ion pump. FIG. 1 shows a typical diode sputter ion pump 10 consists of two cathodes 12, one on either side of an anode 14. FIGS. 2, 3 and 4 show cross-section of prior art anodes of differing design. Each anode typically includes multiple anode cells 16, each having a longitudinal axis perpendicular to the planes of the cathodes. In operation, a positive voltage is applied to the anode 10, a negative voltage or ground potential is applied to the cathodes 12, and a magnetic field is applied parallel to the longitudinal axes of the anode cells. Within each anode cell 16, electrons are trapped by the magnetic field, creating a stable electron cloud commonly known as a space charge cloud.
The electron cloud is stable because the applied magnetic field constrains the electrons to travel in circular orbits each having a radius known as the cyclotron radius. Moreover, at higher pressures, individual electrons are shielded from the electric field of the anode, through a phenomenon known as Debye screening, by other electrons in the cloud. The distribution of voltages and electrical charges in the system creates near the anode an area of steep potential gradient known as the anode sheath. The anode sheath tends to act as a boundary between the edge of the space charge cloud and the anode. The electrons tend to remain in the cloud until they migrate to the anode, where they are counted as anode current.
Electrons in the cloud collide with and ionize gas molecules that migrate into the cloud. The ionized gas molecules accelerate toward the cathodes 12, sputtering cathode material, typically titanium. The sputtered titanium strikes and adheres to the anode, the cathodes, or elsewhere. Because the titanium is chemically active, gas molecules stick to and/or react with the titanium atoms, and are thereby fixed into a solid state and removed from the gas phase thus lowering the gas pressure in the vacuum chamber, essentially pumping gas from the chamber to create a better vacuum. Noble gas molecules that are not chemically active are removed from the gas phase by being buried under sputtered cathode material or by migrating into the crystal structure of the cathode after impact and being trapped within crystal structure defects in the cathodes.
The pumping characteristics of an ion pump are determined primarily by the gas pressure in the vacuum chamber, the magnetic field, the voltages on the anode and cathodes, the shape of the anode cells, the distances between the anode cells and the cathodes, and the types of gases present. The pump cells are characterized by a sensitivity, which is defined as the ion current divided by the pressure and generally given in amps per Torr. The pump is generally characterized by a pumping speed which varies with the particular gas being pumped because of the different chemical reactions between the sputter cathode material and the particular gas molecule. The pumping speed is generally given in liters per second.
When a gas molecule is ionized by collision with an electron in the anode cell, one or more electrons are freed into the electron cloud. To maintain a steady state, electrons must leave the electron cloud at the same rate that new electrons are added to the cloud by the ionization of gases or by the arrival of secondary electrons due to ion bombardment of the cathode. An excess of electrons in the electron cloud will neutralize the gas ions before they have gained sufficient momentum to efficiently sputter material from the cathode.
By a phenomenon known as cross-field mobility, some electrons penetrate the anode sheath and impact the anode. Electrons in the space charge cloud within about two electron cyclotron radii of the sheath have a significant probability of striking the anode and leaving the cloud. The shape of the anode cell has a significant effect on the contour of the anode sheath and its distance from the anode wall, which contour and distance are also affected by the pressure, the magnetic field, and the applied voltages.
The ion pump anode of FIG. 2 is constructed as a series of rectangular cells, as described, for example, in U.S. Pat. No. 3,319,875 to Jepsen. The anode sheath does not conform well to the walls of a rectangular anode cell at the normal operating pressures of the ion pump, causing the anode sheath to be positioned away from the wall over much of the cell. Because the distance from the edge of the space charge cloud to the anode in many parts of its orbit is beyond the cyclotron radius, electrons in orbit around the edge of the space charge cloud do not have a high probability of striking the anode. Thus, the square cell anode is intrinsically inefficient, that is, has a low sensitivity, and square cell anodes have therefore been largely abandoned in favor of anodes that include a gathered cluster of cylindrical sectors as shown in FIGS. 3 and 4.
In a cylindrical cell anode, the edge of the space charge cloud more closely follows the contour of the anode and therefore more electrons can be within the cyclotron radius of the anode while the parameters that determine the sheath, such as magnetic field, pressure, and voltage, are also conducive to effective pumping. The cylindrical cell maximizes the opportunity of the electrons to make their way to the anode itself, which is in close proximity to the space charge cloud. Thus, ion pumps having cylindrical diode cells are more sensitive than ion pumps having rectangular cells.
Diode sputter ion pumps having cylindrical cell anodes, however, display instabilities typically following pumping exposure to gas doses greater than the ultimate pressure of the vacuum system in which the pump is operating. The instabilities include current bursts, leakage currents, and arcs. The instabilities are disruptive to the devices to which the sputter ion pump is attached. For example, a current burst may stimulate a high voltage discharge that disrupts the electronic sub-systems of the system in which the pump is used. Such disruptions are a known cause of system failure.
Thus, it is an object of the invention to enhance the operational stability of ion pumps.
Another object of the invention is to enhance the stability of systems into which ion pumps are incorporated.
A further object of the invention to produce an efficient ion pump anode that minimizes or eliminates instabilities caused by the inter-cylindrical cells.
Yet another object of the invention is to minimize or eliminate instabilities caused by the inter-cylindrical cells by eliminating or minimizing the inter-cylindrical cells.
Still another object of the invention is to produce an ion pump anode having a cell configuration that minimizes or eliminates inter-cylindrical cells yet allows for efficient transfer of electrons from the electron cloud to the anode.
Yet a further object of the invention is to provide a method of efficiently manufacturing a stable ion pump.
In a prior art cylindrical anode ion pump as shown in FIG. 3 between each cylinder its nearest neighbors are inter-cylindrical cellular regions, typically having the shape of a hyper-extended square. One of the applicants has discovered that these inter-cylindrical cellular regions or cells contribute to instabilities and are a liability to the clean and quiet operation of the diode sputter ion pump. The inter-cylindrical cells have been found, by one of the applicants, to support a very dense plasma, which encourages the growth of dendrites on the cathode below the inter-cylindrical cell.
Both the size of dendrites and the number of dendrites per unit area on the cathode surface has been found to be greater under the inter-cylindrical cells than elsewhere on the cathode. Such dendrites cause explosive cathode arc emission and field electron emission from the cathode plate, which are responsible for such instabilities as current bursts and leakage currents.
The present invention relates to a sputter ion pump that minimizes or eliminates instabilities caused by the inter-cylindrical cells. The instabilities caused by the inter-cylindrical cells can be eliminated by eliminating or minimizing the inter-cylindrical cell, or by altering the inter-cylindrical cells so that they do not support a dense plasma. A preferred anode cell design eliminates the inter-cylindrical cells entirely, while maintaining substantial conformance of the anode sheath to the anode cell wall to allow electrons leave the electron space charge cloud.
A preferred anode cell is quasi-cylindrical, that is, it approximates a cylinder to the extent consistent with eliminating the inter-cylindrical cell. For efficiency, the difference between the major and minor axes of the quasi-cylinder should preferably be less than or equal to approximately two electron cyclotron radii. The curved walls of the present invention allow the anode sheath to conform sufficiently to the anode wall so that electrons can efficiently leave the electron space charge cloud and strike the anode, while the quasi-cylindrical shape allows the anodes to fill the space of the anode without creating inter-cylindrical cells. Anode cells of the present invention are non-rectangular, thereby eliminating the inefficiencies inherent in prior art rectangular cell anodes.
The present invention also includes a method of making the inventive anode by connecting curved metal strips to form the anode cells. The present invention also encompasses a charged particle beam system that uses the inventive ion pump and therefore exhibits increased stability.
Additional objects, advantages and novel features of the invention will become apparent from the detailed description and drawings of the invention.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.